A modem is an electronic device that incorporates both a modulator and a demodulator into a single piece of signal conversion equipment. Interfacing directly to a communication channel, modems establish communication links between various computer systems and terminal equipment. The International Telegraph and Telephone Consultative Committee (CCITT), which determines protocols and standards for telephone and telegraph equipment, has authored a number of recommendations describing modem operation. See CCITT Data Communication Over the Telephone Network Series V Recommendations, BLUE BOOK Volume VIII-Fascicle VIII.1 Melbourne, 1988. In most cases, the communications channel is the general switched telephone network (GSTN) or a two-or four-wire leased circuit. Originally, these channels were assigned for voiceband transmission, so they are bandlimited from 300 hz to 3400 hz. The CCITT V.32. bis recommendation specifies the symbol rate of 2400 bauds per second. See CCITT Recommendation V32.bis Geneva, 1991. To achieve data transmission rates of 7200, 9600, 12000, and 14400 bits per second, three, four, five or six bits must be transmitted in one symbol interval. By adding a single bit to a binary symbol with K bits, the number of waveforms to be produced by the modulator is increased from 2k+2k+1. To achieve the same value of the probability of error, an increase in alphabet size within the same bandwidth requires a 3 dB increase in the signal to noise ratio (SNR).
Traditionally, a modem was implemented using analog discrete components. Today, digital circuits centered around one or two high performance digital signal processors (DSPs) can meet the demands of modem methods without the difficulties associated with analog circuitry. A digital modem implementation offers programmability, realizability of sophisticated methods, temperature insensitivity, ease of design, and often reduced cost when compared with analog implementations.
Modem communication technology has advanced to enable an increase in data communication rates. In order to achieve higher communication rates, communication bandwidth usually expands. But, because phone lines are bandlimited, new techniques for effective data communication rate improvements have emerged.
Another of the CCITT telecommunication standards is the V.34 modem 33.6 kbps. See ITU Recommendation V.34, xe2x80x9cA modem operating at data signalling rates of up to 33,600 bit/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuitsxe2x80x9d, Geneva, Oct. 9-18, 1996. Such variable data rates are made possible by using advanced signal processing techniques such as channel coding using 4D convolutional codes (see L. F. Wei, xe2x80x9cTrellis-Coded Modulation with Multi-dimensional Constellations,xe2x80x9d IEEE Transactions on Information Theory, vol. IT-33, no. 4, July 1987.), shell mapping (see R. Laroia, N. Farvardin, S. A. Tretter, xe2x80x9cOn Optimal Shaping of Multidimensional Constellationsxe2x80x9d, IEEE Transactions on Information Theory, vol. IT-40, no. 4, July 1994.), preceding (see R. Laroia, xe2x80x9cCoding for Intersymbol Interference Channelsxe2x80x94Combined Coding and Precoding,xe2x80x9d IEEE Transactions on Information Theory, vol. IT-4, no. 4, July 1996.), nonlinear encoding (see W. L. Betts, E. S. Zuranski, xe2x80x9cMethod and Apparatus for Adaptively Providing Precoding and Preemphasis conditioning to Signal Data for Transfer over a Communication Channel,xe2x80x9d U.S. Pat. No. 5,396,519, March 1995.) and pre-emphasis filtering at the transmitter (see W. L. Betts, prior).
The 4D convolutional codes yield a coding gain above 4.2 db, and an associated Viterbi subset decoder has reduced complexity due to the lower number of state transitions between the states. Shell mapping is a technique for non-equiprobably signaling that reduces the transmitted signal power and thereby yields a coding gain of 0.7-0.9 dB. The precoder implements decision feedback equalization at the transmitter in a way that creates a valid signal for the Viterbi subset decoder at the receiver. Finally, an intelligent choice of the pre-emphasis filter reduces the transmitted signal power at the low frequencies and hence, the level of nonlinear harmonics in the network echo is reduced for a higher level of cancellation.
In the receiver, after demodulation, the baseband signal is subjected to timing recovery, carrier recovery, fractionally-spaced equalization, followed by the Viterbi decoder which decodes the 4D codes of Wei. The Viterbi subset decoder is the most computationally expensive block in the receiver, and the complexity depends on the 16, 32, or 64 state codes being used.
The Viterbi subset decoder (and the equalizer in the self-learning mode) require the knowledge of the closest hard point on a constellation. When precoding is enabled there is constellation expansion, and the point slicing method, which yields the closest hard point on the constellation, assumes that the hard points lie on the infinite lattice. See M. V. Eyuboglu, xe2x80x9cFlexible Precoding for V. FAST,xe2x80x9d Int. Conf. on Data Transmissionxe2x80x94Advances in Modem and ISDN Technology and Applications, 1992, Vol. 356, Ch.26, No.356, pp.13-18, and also G. D. Forney, Jr., xe2x80x9cAdvances in Modem Technology since V.32/ V.32bis,xe2x80x9d Int. Conf. on Data Transmission xe2x80x94Advances in Modem and ISDN Technology and Applications, 1992, Vol.356, Ch.26, No.356, pp.1-6. A common method for determining the hard points is the Infinite Point Slicing (IPS) method which has a simple implementation. When the constellation expansion is high compared to the constellation size, the distance between the constellation points is effectively reduced in order to maintain the constant transmitted signal power. Therefore, the precoding is disabled in this case, which can result in inter-symbol interference (ISI) caused by severely distorted channels. Consequently, the point slicing method must yield hard points from the particular finite size constellation used in the transmitter of the remote modem and be capable of distinguishing the symbols from one another to yield the correct hard point from among the hard points in the finite size constellation.
Presently, there are no computationally efficient methods for yielding hard points in a 4-quadrant finite constellation that is generalized for all constellation configurations in the V.34 modem standard. The IPS is a computationally efficient method, however, it generates erroneous hard points when it is necessary to consider the constellation boundary. The necessity for a method that does not generate erroneous hard points when it is necessary to consider the constellation boundary is stated in Wei.
In at least one previous work, Martinez, K., Mack, G., xe2x80x9cViterbi Decoder for Wireline Modems,xe2x80x9d U.S. Pat. No. 4,709,377, (1985), a point slicing method was presented. In that work, the QAM constellation that was assumed is not general in the sense that it has a rectangular shape in the first quadrant. The received soft points are therefore translated into the first quadrant, the nearest hard point is found in the first quadrant, then the hard point is reflected into the quadrant of its corresponding soft point. Due to the rectangular nature of the constellation boundary, the bounds of the perimeter in the first quadrant are straight lines, and the soft points outside this perimeter are shifted inside the perimeter by truncating the real or imaginary parts.
The present invention addresses and solves the problems of selecting erroneous hard points found in the prior art by determining a valid hard point constellation and projecting received external soft points into the constellation boundary to locate the closest valid hard point. The present invention provides a point slicing method for digital transmission based on QAM (Quadrature Amplitude Modulation) constellations. Upon receiving noisy constellation points at the equalizer output, such as shown in FIG. 3 for the V.34 modem, the proposed method obtains the closest valid constellation hard point in each subset. The computational complexity of the proposed method is nearly independent of the constellation size. The present invention finite point slicing (FPS) method is based on modifying the IPS method and conducting a search over a maximum of nine candidate hard points, when necessary. The invention FPS method does not generate erroneous hard points when it is necessary to consider the constellation boundary. This invention pertains to the receiver section of a modem used in data communications, and/or particularly, to a receiver incorporating a Viterbi decoder without the need for special processors dedicated for trellis decoding.
The present invention is a computer method and apparatus of point slicing in digital communication systems employing QAM having a defined number of bits per symbol, wherein the number of bits per symbol defines a QAM hard point constellation. The invention method comprises the steps of (i) providing an infinite lattice having a coordinate system and an origin and having at least one subset of QAM constellation hard points, (ii) receiving at least one subset of soft points where these soft points spatially map onto the infinite lattice, (iii) defining at least one valid constellation boundary where this boundary defines a set of valid constellation hard. points, (iv) determining a closest valid constellation hard point for each received soft point, and (v) reporting the closest valid constellation hard point.
The invention method for determining a closest valid constellation hard point comprises the steps of (i) defining at least one valid constellation boundary for selecting a finite constellation of hard points from the infinite hard point constellation, and (ii) the step of determining a closest valid constellation hard point comprises the steps of selecting from the received soft points multiple subsets of soft points including a first subset of soft points, locating a closest valid finite constellation hard point to each soft point in said first subset of soft points, and locating a closest valid finite constellation hard point for each soft point in all other subsets of soft points. The step of locating a closest valid constellation hard point includes the steps of (i) comparing each soft point location with the finite constellation boundary, (ii) locating a closest hard point for each interior soft point, and (iii) locating a closest hard point for each exterior soft point. The step of locating a closest hard point for each exterior soft point further comprises the steps of (i) projecting each exterior soft point along an imaginary line connecting to the constellation origin to get an associated intermediate soft point for each exterior soft point, and (ii) locating a closest intermediate hard point for each intermediate soft point, and (iii) searching all hard points surrounding and including said closest intermediate hard point for each intermediate soft point, where the final hard point is a member of the valid finite constellation hard points. Projecting each exterior soft point comprises the steps of defining a plurality of distant ratios, and multiplying each exterior soft point coordinates by corresponding distance ratios to project the exterior soft point inside of and including onto the finite constellation boundary. Defining a plurality of distance ratios includes computing the equation:       (                  I        s        p            ,              Q        s        p              )    =                              d          max                -        ε                    d        ⁡                  (                                    I              s                        ,                          Q              s                                )                      xc3x97                  (                              I            s                    ,                      Q            s                          )            .      
Selecting a finite constellating boundary comprises the step of defining a geometric shape about the constellation origin to determine a spacial boundary about the constellation origin. Selecting a finite boundary comprises the step of passing the geometric shape through the finite constellation hard point located farthest from the constellating origin. Finally, locating the valid finite constellation hard points for each soft points in all other subsets of soft points comprises the steps of performing a soft point coordinate system transformation with each subset coordinate system to the first subset coordinate system, locating a closest valid finite constellation hard point to each soft point in each subset from among the hard points in the first subset coordinate system, and performing a hard point coordinate system transformation to map each located closest hard with each soft point subset into the subset original coordinate system.
The invention apparatus is a digital communication device for receiving QAM signals that employs a decoder for decoding received soft points into corresponding hard points in a QAM constellation. The invention apparatus comprises (i) a signal processor, (ii) a QAM constellation boundary routine to define a set of valid QAM constellation hard points, and (iii) a finite point slicing routine operating on the signal processor that locates corresponding valid QAM constellation hard points for the received soft points.